1. Field of the Invention
The present invention relates generally to enhancing the performance of a project and more particularly to enhancing the acoustical performance of a structure.
2. Background Information
Sound is an energy which is generated by a source, transmitted through a medium, and received by a receiver. It is measured by its power pressure level at various frequencies, typically represented by octave and ⅓ octave bands. The magnitude of the sound's power and pressure level for a given bandwidth is measured and expressed in units called decibels (dB). The higher the decibel level, the greater the intensity of the sound being transmitted for the bandwidth being measured.
The human ear is more sensitive to some frequencies than to others. The ear perceives a 1000 Hertz (Hz) tone as louder than a tone at 200 Hz or 4000 Hz, even though all three tones may have the same decibel level. Table 1 relates sound levels to common sound sources.
TABLE 1Examples of Sound Sources and Associated Human ResponseSourcedBSensory ResponseThreshold of good hearing10Very faintWhispered conversation at 6 ft.30FaintConversational speech at 3 ft.60ModerateComputer printout room80LoudLoud rock band110Threshold of discomfortPassenger ramp at jet airliner120Threshold of painMilitary jet takeoff at 100 ft.140Extreme danger
A person's response to a sound is dependent on changes in sound pressure level, the spectral content and temporal variation of the sound source, and any background noise that may be present. The human ear does not perceive sound changes in direct proportion to the change in decibel levels. For example, to perceive a halving or doubling of loudness, the sound pressure must be changed by about 10 dB. Table 2 demonstrates the relationship between sound power level changes and human perception.
TABLE 2Sound Power Level Changes and Human PerceptionSound Power Level DecreasePerception0-3dBBarely perceivable4-5dBPerceivable and significant6dBResultant sound level ¾ of initial level7-9dBMajor perceived reduction10dBResultant sound level ½ of initial level
The ability of a structural component (e.g., a wall or a door) to block or absorb sound transmission from one room to another is often represented as a single-number rating called Sound Transmission Class (STC), as described by the American Society For Testing and Materials (ASTM). This rating is calculated by measuring in decibels the transmission loss at several frequencies under controlled test conditions and then calculating the single-number rating from a prescribed method. Table 3 provides an indication of the effectiveness of various STC ratings in reducing the speech transmission from one room in a structure to an adjacent room.
TABLE 3STC Ratings and Approximate Speech Transmission ReductionSTCPrivacy Afforded25Normal speech easily understood30Normal speech audible, but unintelligible35Loud speech understood40Loud speech audible, but unintelligible45Loud speech barely audible50Shouting barely audible55Shouting not audible
When an actual constructed system is concerned, however, where conditions such as humidity and interior volume are not controlled in a laboratory environment, the single-number rating describing the acoustical performance of such a system can be expressed as a field STC rating (FSTC), which approximates a STC rating. The higher the FSTC rating of a constructed system, the greater the transmission loss.
In building modern residential structures, such as single-family houses, an important factor to consider is noise control. In order to provide a livable and quiet environment, sounds created by sources such as televisions or conversation must be controlled and reduced to comfortable decibel levels. In the case of commercial structures, such as offices and hotels, builders often have available to them complete acoustical performance packages which have resulted from years of experience and testing. Such packages are able to achieve high and consistent FSTC ratings, largely because the structures involved are usually of such typical geometry and configuration as to allow for the successful re-application of tried-and-true methods from one building to the next. However, for builders of residential structures, which can vary considerably in layout, such proven, complete, and economical system packages are not available.
In addition, materials used in commercial construction generally provide higher sound transmission loss levels that those used in residential construction. With the use of such elements as concrete floors, floors with gypcrete toppings, and resilient steel studs in wall assemblies, sound transmission loss is not compromised by any one floor or wall assembly. Also, with the specification of such components as acoustical doors (e.g., for private conference rooms), opportunities are few where a high-performing component is installed with a low-performing component. In contrast, materials and components used in typical residential construction result in systems made up of components performing at equally low transmission loss levels. Because of this, the acoustical performance levels of all of the components (e.g., floors, walls, and doors) have to be improved to an approximately equal level in order to achieve a substantial acoustical benefit.
Currently, two main options are available to builders for achieving some amount of noise control in residential structures. One method involves an acoustical upgrade by an insulation subcontractor or another type of subcontractor. Production builders, i.e., non-custom builders, may offer home buyers an acoustical insulation upgrade as one of the options associated with constructing a single-family structure. Such an upgrade is normally installed by an insulation subcontractor concurrently with the required thermal insulation. Some acoustical upgrades involve a simple solution applied to an entire structure, but most apply to limited areas of a house, such as bedrooms, laundry rooms, or bathrooms. Acoustical upgrades are also sometimes offered in packages of varying price, each commensurate with the amount of work to be performed. For example, a “good” option may only include fiber glass sound control insulation in interior walls and some floors. A “better” option could further include the caulking of gaps and perimeter joints in walls, which are normally left untreated. For a “best” option, an insulation contractor in conjunction with the drywall contractor may also install resilient channels between the studs and the drywall, or joists and a gypsum ceiling.
The above scenario results in a residential structure that has slightly improved acoustical isolation between rooms. The performance improvement could be described in terms of A-weighted sound pressure level (dBA), and an expected improvement using the described method might be 1-10 dBA, depending on what components are measured and where. Although some home buyers may perceive that the rooms within their house have been “sound deadened,” the current good/better/best package upgrades do not actually provide significant measurable benefits.
The reason for this is because all of the rooms still contain several acoustical weak links or acoustical short circuits that the currently offered acoustical upgrades do not address. As mentioned above, the performance levels of all the components used in building a residential structure have to be improved to an approximately equal performance level in order to achieve a substantial acoustical benefit. Current upgrades, in contrast, only address segments of a system, such as doors or walls, as opposed to an entire system (i.e., a combination of segments). In room-to-room acoustics, one can only achieve a FSTC rating slightly better than the performance of the weakest link; sound follows the path of least resistance. For example, a wall built with resilient channels and fiber glass batts may provide an STC of 42 (in a controlled environment), but if that wall contains a typical solid core or hollow core interior door with a STC of 20, then the wall plus door will only yield an overall STC of around 21-24. Similar weak links occur with electrical outlets, background noise, HVAC ducts, structural flanking, and plumbing. Flanking paths (i.e., sound leakage) can be present even when the surrounding construction is of good quality. Direct HVAC ductwork and direct electrical outlet positioning between rooms and corridors also create escape routes for sound (i.e., cross-talk).
The other method for enhancing acoustical performance in a residential structure involves an acoustical expert, whom a custom builder relies on for advice. An acoustical expert will typically review the building plans and systematically inspect areas of house, detecting most of the weak links and addressing them using textbook principles and previous experience. In many cases, acoustical experts are able to correct many of the weak links, but academic principles usually fail to factor in the interactions of the “real world,” e.g., location and type of framing members, short circuits, leaks, etc. Complete acoustical enhancement systems that are both practical and economical are not contained within reference texts. Therefore, an acoustical expert would likely overdesign some of the necessary acoustical components and overcome other obstacles with extra mass and expensive solutions.
Presently, no complete, practical, and economical acoustics package system, balancing solutions to all possible weak links, is available for the production and semi-custom home segments.